1. Field of the Invention
The present invention relates to a metal powder suitable for producing an anode body for an electrolytic condenser, an electrolytic condenser anode body that uses the powder, and an electrolytic condenser.
2. Description of the Related Art
FIG. 1 is a perspective view showing an example of an electrolytic condenser anode body. Reference symbol 1 in the drawing is an anode sintered body, and an anodic oxide film is formed thereon. Reference symbol 2 is a lead wire, and the lead wire 2 is partly embedded in the anode sintered body 1.
In producing the electrolytic condenser anode body, a metal powder of tantalum or niobium, etc. and the lead wore 2 are first placed in a mold and pressure molded followed by sintering. As a result, the metal powder is pressed and sintered resulting in the formation of porous anode sintered body 1, and the anode sintered body 1 and lead wore 2 are integrated into a single unit. An anodic oxidation treatment is then performed on anode sintered body 1 to form an anodic oxide film that results in the obtaining of an electrolytic condenser anode body.
An electrolytic condenser is obtained by forming a solid electrolyte layer, for example, manganese dioxide or lead oxide, a graphite layer and a silver paste layer on the anode sintered body 1 of the anode body which was obtained in the manner as described above, and covering them by a plastic casing after connecting a cathode terminal and anode terminal.
The anodic oxidation treatment for forming the anodic oxide film on the anode sintered body 1 is carried out by immersing the anode sintered body 1 in an anodic oxidation treatment solution of, for example, H3PO4, and applying a voltage. In the present specification, the voltage applied at this time is referred to as the anodic oxidation treatment voltage (or simply as the treatment voltage). During the anodic oxidation treatment, an oxide that results in the formation of an oxide film is formed on the portion that contacting with the anodic oxidation treatment solution, of the metal powder surface that composes the porous anode sintered body 1.
For example, a tantalum powder that is used for the electrolytic condenser is obtained by reducing potassium tantalate fluoride with sodium, heat treating the resulting primary particles to aggregate the particles followed by crushing and sizing to a suitable particle size range.
During the anodic oxidation treatment, as schematically shown in FIG. 2. for example, an oxide film 12 is formed by the formation of oxide (Ta2O5) due to consumption of the surface portion of tantalum powder 11. The higher the anodic oxidation treatment voltage, the greater the thickness of the oxide film 12.
In a tantalum electrolytic condenser, the interface between the oxide film 12 and the portion inside that is not oxidized (also referred to as metal tantalum) 13 contributes to charge accumulation. Thus, when considering, for example, a single tantalum powder 11, when the entire tantalum powder 11 becomes an oxide due to anodic oxidation treatment, and the portion of the metal tantalum 13 in the center is no longer present, a condenser function is not demonstrated by that tantalum powder 11.
However, increasing the total surface area of the metal powder in the anode sintered body 1 is effective for enhancing the capacitance of the condenser, and in order to accomplish this, powder having a small particle size and large specific surface area has been preferentially used for the tantalum powder 11 for the condenser. In addition, condensers have become increasingly small in recent years in accommodation of the reduced size and more sophisticated functions of electronic equipment, and in response to this, hyperfine powder has come to be used for the tantalum powder 11 for the condenser.
When hyperfine tantalum powder 11 is used, thickness of the anodic oxide film 12 approaches the particle radius of the tantalum powder 11. Since the ability to accumulate charge is lost if all of the tantalum powder 11 is converted to oxide as described above, it is not possible to obtain a condenser having high electrostatic capacitance.
However, since emphasis was primarily placed on reducing the particle size of the tantalum powder, while the relationship between the tantalum powder particle size and anodic oxidation voltage was viewed with little importance, condensers having high electrostatic capacitance were not always obtained.
For example, use of the tantalum powder having an agglomerated particle size of 250 xcexcm or less and BET specific surface area of 0.42 m2/g is disclosed in Japanese Unexamined Patent Application, First Publication No. 4-218608. The anodic oxidation treatment voltage here is 70 V.
In addition, in Japanese Unexamined Patent Application, First Publication No. 4-136102, the tantalum powder having a BET specific surface area of 0.2 5 m2/g (equivalent to 1.4 xcexcm in terms of sphere diameter) is used, and the anodic oxidation treatment voltage is set to 100 V.
Moreover, use of the tantalum powder having a mean particle size of 130 nm and carrying out anodic oxidation treatment at a treatment voltage of 40 to 140 V is disclosed in Japanese Unexamined Patent Application, First Publication No. 8-97095.
On the other hand, from the viewpoint of achieving power saving in compact equipment in an attempt to prolong battery life, it has been proposed to lower the anodic oxidation treatment voltage, and recently, low anodic oxidation treatment voltage on the order of 5 V has come to be used.
In consideration of the above circumstances, the object of the present invention is to optimize the relationship between the anodic oxidation treatment voltage and particle size of the metal powder for the electrolytic condenser so as to form an anodic oxide film of a suitable thickness with respect to the particle size of the metal powder and obtain a condenser having high electrostatic capacitance.
In order to achieve the above object, the present invention provides a metal powder for an electrolytic condenser which is used to produce an electrolytic condenser anode body wherein, when the anodic oxidation treatment voltage when forming an anodic oxide film on an anode sintered body composed of the metal powder is Vf (units: V), particles in which primary particle diameter as determined by image analysis is within the range of 2.7xc3x97Vf to 10xc3x97Vf (units: nm) are contained at 50 wt % or more.
The electrolytic condenser anode body of the present invention comprises pressure molding the metal powder of the present invention with a lead member followed by sintering and forming an anodic oxide film on the resulting anode sintered body.
The electrolytic condenser of the present invention is equipped with the electrolytic condenser anode body of the present invention as the anode.
A production method of the electrolytic condenser anode body of the present invention comprises presetting the anodic oxidation treatment voltage to Vf (units: V), and pressure molding the metal powder for the electrolytic condenser containing 50 wt % or more of particles, in which the primary particle diameter as determined by image analysis is within the range of 2.7xc3x97Vf to 10xc3x97Vf (units: nm), with a lead member, followed by obtaining a sintered body by sintering and performing anodic oxidation treatment using the anodic oxidation treatment voltage Vf on the anode sintered body to form an anodic oxide film. The above anodic oxidation treatment voltage Vf can be preferably set within the range of 5 to 250 V.